Our goal is to understand the determinants of ventilatory muscle endurance. The general hypotheses which will be tested are 1) that endurance depends on mean airway pressure, inspiratory flow rate, breathing frequency, duty cycle and lung volume, each as independent variables, 2) that endurance depends on the rate of energy utilization, and 3) that there are distinct responses of endurance to training of ventilatory muscles by specific tasks (high flow, low pressure versus low flow, high pressure). Endurance testing will be performed by having normal subjects breathe against inspiratory loads until they can no longer sustain the assigned task. Tasks will be assigned with different intensities and with different combinations of pressure, flow rate, duty cycle, frequency, and mean lung volume. These experiments will be interpreted in terms of basic muscle properties and chest wall configuration. Oxygen consumption as an index of energy utilization will be related to these variables during both fatiguing and nonfatiguing tasks. Oxygen consumption will also be related to endurance both for a range of tasks at a given endurance and a range of endurances. Subjects endurances for specific tasks (high flow, low pressure and the converse) will be measured before and after a 6 week period of daily training with specific inspiratory tasks. Characterization of the performance of the respiratory system and understanding of the mechanisms of its failure during fatiguing loads and its response to training are highly relevant to acute and chronic respiratory failure.